The invention relates to symbols used to prepare a presentation for display for traffic surveillance and collision avoidance systems, for example, in aircraft flying in a formation.
Presently, most aircraft utilize systems that provide pilots information to avoid potential collisions in the air and/or on the ground. There are many varieties of collision avoidance systems (CAS) and conflict detection systems in aircraft. Generally, typical modes of operation fall into the following categories: (1) passive modes of operation; and (2) active modes of operation. A system operating in a passive mode collects information about the position of nearby aircraft by receiving unsolicited messages (e.g., squitters) and by eavesdropping on the replies to the interrogation signals generated by other nearby systems that are operating in an active mode. Active collision avoidance systems transmit signals from the host aircraft to determine relevant information about nearby aircraft, and/or to provide information about the host aircraft to nearby aircraft. The most prevalent active system used in the U.S. today, is the Traffic Alert and Collision Avoidance System (TCAS) as described, for example, in DO-185A xe2x80x9cMinimum Operational Performance Standards for Traffic Alert and Collision Avoidance System II (TCAS II)xe2x80x9d available from RTCA Inc. TCAS is internationally known as Airborne Collision Avoidance System (ACAS).
TCAS offers pilots of civil and military aircraft reliable information to track traffic and avoid potential collisions with other aircraft. A conventional TCAS installation in an aircraft includes several airborne devices that cooperate. These devices generally operate independently of ground-based Air Traffic Control (ATC) systems. Since TCAS inception, three different control levels have evolved: TCAS I is intended for commuter and general aviation aircraft and provides a proximity warning only, assisting the pilot in visually acquiring intruder aircraft; TCAS II is intended to provide pilots with traffic advisories and resolution advisories in the vertical plane; and TCAS III, which has yet to be approved by the FAA, is intended to provide resolution advisories with horizontal as well as vertical flight paths. TCAS as used herein includes any of these control levels.
TCAS transmits interrogation signals (e.g., ATCRBS Mode C or Mode S signals) and detects the presence of nearby aircraft equipped with transponders that reply to the interrogation signals. When nearby aircraft are detected, TCAS tracks and continuously evaluates the potential of these aircraft to collide with the host aircraft.
For surveillance, TCAS interrogation signals (i.e., interrogations) are transmitted over an interrogation channel (e.g., 1030 MHz) from the TCAS equipped host aircraft. Each interrogation requests a reply from one or more transponder-equipped aircraft within range of the host aircraft. The reply or replies typically include pertinent position and/or intent information of the replying aircraft. Transponder-equipped aircraft within range of the transmitted interrogation reply over a reply channel (e.g., 1090 MHz). The reply may further include altitude, position, bearing, airspeed, aircraft identification, and other information of the replying aircraft to assist the TCAS on the host aircraft in tracking and evaluating the possibilities of collision between the host aircraft and the replying aircraft.
TCAS performs surveillance, tracking, and collision avoidance advisory functions. In operation, a symbol depicting each nearby aircraft is presented on a display located in the cockpit. The displayed symbols allow a pilot to maintain awareness of the number, type, and position of nearby aircraft. An aircraft that is (or is about to be) too close to the host aircraft is called an intruder. TCAS predicts the time to an intruder""s closest point of approach (CPA) and a separation distance at the CPA by calculating range, closure rate, vertical speed and altitude. TCAS provides the capability of tracking other aircraft within range, evaluating collision potential, displaying/announcing traffic advisories (TAs), and depending on the type of system used (e.g., TCAS II), recommending evasive action in the vertical plane to avoid potential collisions, otherwise known as resolution advisories (RAs).
In certain circumstances aircraft may not be detected by TCAS, for example, aircraft not equipped with operating transponders cannot reply to interrogations; military aircraft equipped with identification friend or foe (IFF) systems operating in mode 4 do not reply to interrogations; and aircraft that may not receive interrogations (e.g., radio interference, mechanical interference such as when the landing gear interfere with an antenna, or when in a mode of operation called interference limiting).
The Federal Aviation Administration (FAA) sets guidelines for collision warning regions and collision caution regions for implementations of TCAS. A volume of space around the host aircraft defines these regions and/or a time to penetration of that space (e.g., generally referred to as tau (xcfx84)). Examples of a collision region 110, warning region 115, and caution region 150 of a host aircraft 105 equipped with TCAS, are illustrated in FIGS. 1A (top view) and 1B (perspective view). If an aircraft 120 penetrates caution region 150 it may be designated as an intruder and a traffic advisory may be issued to the pilot or crew of host aircraft 105. The TA may consist of an audible warning and visual display indicating the distance and relative bearing to intruder 120. If intruder 122 penetrates warning region 115, a resolution advisory may be issued to the crew or pilot of host aircraft 105. The RA may be corrective or preventive and may consist of instructions to climb or descend at a recommended vertical rate or to refrain from making changes in the present vertical rate.
The shapes, horizontal, and vertical dimensions of the respective regions are a function of the range and closure rate of aircraft 120. The time-space domain for TCAS interrogations is limited in that each interrogation-reply takes a certain period of time. When several different aircraft are interrogating in the same proximity, transponder replies may overlap in time (e.g., become garbled). Consequently, air traffic control (ATC) systems may have difficulty tracking individual aircraft. To overcome this problem TCAS was designed with logic that, when a certain number of TCAS equipped aircraft are within a predetermined vicinity of each other, TCAS on each aircraft would operate in an interference limiting mode having reduced output power, reduced number of interrogations, reduced receiver sensitivity, and consequently a reduced intruder tracking range. In low traffic density regions increased transmission power is suitable whereas in high traffic density regions (conventionally called Terminal Control Areas (TCAs)) reduced transmission power is desirable. For example, the TCAS of an aircraft flying over Western Kansas may have an interrogation range of 80 nm (nautical miles) or longer; whereas, an aircraft flying near Chicago may reduce its interrogation range to 5 nm with greater link margin. The reduction of transmission power from a low density region to a high density region may be as much as 10 dB. Transmission power is reduced to reduce RF interference between TCAS equipped aircraft and to reduce RF interference with ATC ground tracking stations.
If TCAS equipped aircraft, such as military aircraft, were to fly in a multi-aircraft group known as a formation, and each TCAS was actively interrogating, each airborne TCAS of an aircraft included in the formation and those nearby but not in the formation may react to the seemingly high density of traffic and begin operating in the interference limiting mode. Each may also reduce receiver sensitivity to compensate for the perceived density. The resulting reduction in intruder tracking range would increase the risk of collision to unacceptable levels (e.g., particularly with aircraft flying at relatively high speed). TCAS equipped aircraft may begin operating in interference limiting mode even in formations of two or three aircraft.
Honeywell (formerly Allied Signal) developed a collision avoidance system designed to specifically address military formation-flying insufficiencies of conventional TCAS; this system is known as Enhanced TCAS (ETCAS). ETCAS provided means for military planes to fly in formation by offering a rendezvous-type feature in collision avoidance systems that would allow aircraft to be able to fly in a formation with other aircraft without generating RAs and TAs against one another. However, ETCAS also generated significant interference limiting behavior in non-formation aircraft. The FAA and civilian regulatory agencies of other countries severely restricted the use of TCAS, including ETCAS, during formation flying due to the consequences of inappropriate operation in an interference limiting mode. These restrictions essentially require several members in a formation to fly with their TCAS turned off, while one or a few aircraft in the formation are allowed to fly with TCAS turned on. These restrictions obstruct the purpose of collision avoidance systems since many members of a formation have no indication of potential collision threats between themselves and non-formation aircraft as well as potential collisions threats between other members of the formation. Further, the restrictions on the use of TCAS during formation flying detract from the advantages of using ETCAS.
The block diagram of FIG. 2 illustrates an example of interference limiting. As shown, a group of aircraft 210-215 are flying in formation 200 while TCAS equipped aircraft 220 is approaching formation 200. The wavy lines in front of an aircraft symbol in FIGS. 1-3 indicate transmission from the TCAS aboard that aircraft.
When the TCAS of aircraft 220 receives TCAS broadcasts (interrogations) from aircraft 210-214 within range (e.g., within surveillance region 260), the TCAS of aircraft 220 forms intruder tracks and perceives a high density of intruders 210-214. The TCAS of aircraft 220 may consequently begin operating in a mode with reduced surveillance range (e.g., an interference limiting mode), for example, with a surveillance region 261 that is smaller than a typical surveillance region. The reduction in the number and transmission power of TCAS broadcasts is gradual and may not be recognized by a pilot or flight crew. Reducing the size of a surveillance region may be dangerous for aircraft flying at high speeds, as warning time and time to act on a resolution advisory may be significantly reduced.
Presently, under the requirements of the FAA and various other airworthiness authorities in several countries, only one or a few aircraft in a formation is allowed to have an actively interrogating TCAS (referred to herein as active TCAS). If all the members in a formation are not interrogating, significant safety problems can arise. That is, the non-interrogating formation members will not be aware of potential collision threats between themselves and non-formation aircraft because their respective TCAS is turned off. The non-interrogating members of the formation will also have no warning by their respective TCAS of potential collisions with other formation members.
Further, conventional symbols used in presentations of air traffic do not distinguish formation members from non-formation traffic and do not provide indicia of whether or not tracking is based on replies to interrogation. Without symbols of the present invention, unsafe conditions may arise during formation flight including conditions arising from delayed or unnoticed changes in tracking of formation members and non-formation traffic.
The present invention substantially eliminates one or more of the problems associated with the prior art by presenting surveillance and collision avoidance information that distinguish formation members from non-formation traffic and/or distinguish differences in methods for monitoring traffic. Distinctions are made using a set of compound symbols. By presenting symbols for formation members that are different from symbols for non-formation traffic, flight crew can more easily maintain situational awareness, thus increasing flight safety. By presenting symbols for traffic according to replies to interrogations directed to such traffic different from symbols for traffic according to messages received without interrogation, differences in the reliability of the presentation are made evident. Generally, replies to interrogations provide more up to date and more accurate situational awareness to the flight crew than information based on messages received without interrogation. Greater flight safety results from assisting the flight crew to easily maintain up to date and accurate situational awareness.